a prime is a number that yields an integer value when divided by any number other than itself (and 1).

In practice the

test does not have to be run for numbers greater than the square root of the number being tested rounded up to the next whole number.

Also, only other prime numbers need to be taken into consideration (whether applying that is cheaper than a brute force approach is open for debate and implementation dependant).

As a result of the rules, you only need to test odd numbers and need to divide only by odd (prime) numbers smaller than the square root of the candidate prime under consideration.

By taking an array of odd numbers between 100 and 200 and iterating over that building an array of primes found you will get what you want.